The present invention relates to a method of controlling a wire-cut electric discharge machine to prevent cutting errors which would be produced upon flexing of a wire electrode when a corner is cut during an electric discharge cutting operation, and more particularly to a simple method of controlling a wire-cut electric discharge machine to cut an accurate corner.
Wire-cut electric discharge machines operate on the principle that a voltage is applied across a gap between a wire electrode and a workpiece to generate a spark discharge across the gap for cutting the workpiece with the spark energy. The workpiece can be cut to a desired contour by moving the workpiece with respect to the wire electrode based on cutting command data.
More specifically, as schematically shown in FIG. 1, which shows a known wire-cut electric discharge machine, a wire 1 is reeled out of a reel RL.sub.1, extends between a lower guide 4 and an upper guide 4, and is wound around a reel RL.sub.2. A voltage is applied by a contact electrode (not shown) to the wire to generate a discharge between the wire 1 and the workpiece 2 for cutting the workpiece. The workpiece 2 is fastened to a table TB movable by motors MX and MY in the X and Y directions, respectively. Thus, the workpiece 2 can be cut to a desired configuration by moving the table TB in the X and Y directions. The upper guide 4 is attached to a moving mechanism MMC movable by motors MU and MV in the X and Y directions respectively so that the upper guide 4 is movable in the X and Y directions. The moving mechanism, the reels RL.sub.1 and RL.sub.2 and the lower guide 4 are mounted on a column CM.
A numerical control unit NC serves to read the contents of a command tape TP, and has a distributor circuit DS for distributing commands for respective axes and drive circuits SVX, SVY, SVU and SVV for the corresponding axes for energizing the motors MX, MY, MU and MV respectively to move the table TB and the moving mechanism until the workpiece 2 is cut to a desired shape.
FIG. 2 illustrates of a cutting operation of such an electric discharge cutting machine. When the wire electrode 1 moves in and along a slot 3 in a given direction while cutting the workpiece 2 with electric discharge, a pressure is developed between the wire electrode 1 and the workpiece due to the electric discharge and pushes back the wire electrode 1 in the direction of the arrow which is opposite to the direction in which the electrode 1 moves along, as shown in the cross-sectional view of FIG. 3. The wire electrode 1 is therefore backed off or flexes from the position of the wire guides 4. The cutting accuracy is not affected to an appreciable extent by the amount of such flexing as long as the wire electrode 1 cuts the workpiece 2 along a rectilinear slot. However, the amount of flexing causes a serious problem when the wire electrode 1 cuts the workpiece to form a corner. Thus, as shown in FIG. 4, which is a plan view of a cut slot, a slot 3 is composed of a first rectilinear slot L1 and a second rectilinear slot L2 extending perpendicularly to the first rectilinear slot L1, and defining such a combined slot 3 which requires a corner CN to be cut at the junction between the first and second rectilinear slots L1 and L2. To this end, the workpiece 2 and the wire electrode 1 are caused to move relatively in one direction to form the first rectilinear slot L1, and thereafter the direction of such relative movement needs to be changed at a right angle under a cutting command to form the second rectilinear slot L2. The wire electrode 1 however has a tendency to be dragged inwardly of the corner CN due to the flexing of the wire electrode 1 at a position in which the electric discharge takes place, with the result that the contour of the slot 3 as it is cut is distorted considerably inwardly and becomes blunt as shown by the dotted lines, a configuration which is different from a commanded shape (shown by the solid lines).
FIG. 5 is a plan view of an arcuate corner CN' to be formed between the first and second rectilinear slots L1 and L2. In cutting such an arcuate corner CN', the fixing of the wire electrode 1 due to the electric discharge causes the corner CN' to be cut along a path shown by the dotted lines which is duller than a commanded shape as illustrated by the solid lines.
It is known that the cutting errors at such arcuate and angular corners can be reduced by changing the path of cutting, the cutting power supply, the speed of feed, and other factors. However, there are a great many combinations available for such cutting conditions, and the customary practice is complex and impractical as no specific standard is established for controlling the cutting path, the feeding speed, and the cutting power supply voltage.